1. Field of the Invention
The present invention relates to a printing apparatus for and a printing method of forming pixels with dot patterns and more specifically to image processing in a printing apparatus that performs a so-called multipass printing in which the same line is printed in a plurality of scanning operations of a print head each using different nozzles.
2. Description of the Prior Art
As information processing equipment such as computers have come into wide use in recent years, printing apparatus, such as printers, that are peripheral devices of the computers are also becoming rapidly widespread. At the same time as the general trend for higher quality and color representation of visual information in the information processing equipment accelerates, there are also growing demands on the printing apparatus for a higher print quality and for a color printing capability.
A well-known printer image processing method is an image processing technique based on a color halftone model.
FIG. 13 is a block diagram showing an example of a conventional image-processing unit used in a printer.
Image data in the form of an RGB signal is input from an input terminal 101, converted into a resolution level compatible with that of the printer by a resolution conversion unit 102, and then converted into a CMY density signal for each pixel by a density conversion unit 103. The image data, after being converted from the RGB signal into the CMY density signal, is further converted by a black generation unit 104 into a density signal that includes a black density signal K. The image data now in the form of a CMYK density signal is subjected to under color removal processing and masking in a masking/UCR unit 105 whereby the image data is converted into a halftone area signal, a CMYK density signal that has its crosstalk component compensated for. Next, the image data in the form of the halftone area signal is γ-corrected by an output γ-correction unit 106 to compensate for a linearity between the halftone area signal and the output density as by dot gain processing. Then, the image data is converted into binary data (referred to also as “bit map data”) for each color component by a binarization unit 107 and is output from a host side interface unit 108 to a transmission line 201. The bit map data transmitted from the host side interface unit 108 is taken into the printer through a printer side interface unit 109. Further, in an H-V conversion unit 110 the bit map data has its output order converted in accordance with a driving order of a print head. A mask generation unit 111 generates a thinning pattern (mask data) for multipass printing and gives it to a head driver 112, which in turn thins the converted bit map data according to the mask data from the mask generation unit 111. Based on the thinned bit map data, the print head 113 is driven to eject ink during each of the scans with a paper feed operation performed between the scans. An image is thus formed by multipass printing.
As described above, in printing a single scan line the multipass printing involves dividing the bit map data for the same scan line into a plurality of parts and performing a plurality of passes and a predetermined amount of paper feed between the individual passes to eject ink from different nozzles of the print head in each of the passes according to the divided bit map data for the same scan line, thus forming an image for the same line. This multipass printing can reduce unevenness in printing due to dot landing deviations, variations in ink ejection volume and ink penetration time differences.
The density conversion unit 103 and the output γ-correction unit 105 are normally integrated into a lookup table (LUT), rather than being provided individually with processing circuits and software. This arrangement can shorten the processing time.
The conventional image processing method described above, however, has the following problem.
As the resolution of printed images and the printing speed in the printer tend to increase, the amount of image data processed by the image processing unit also increases. With the conventional method, however, the resolution conversion, density conversion, black generation, masking/UCR, output γ-correction, binarization, H-V conversion, and mask processing are all successively performed independently. Hence, processing such a large amount of image data according to the conventional method will take relatively long and the circuit and the amount of calculations required will inevitably become huge.
Further, as the number of dots in a dot pattern that forms a pixel increases, i.e., as the number of gray scale levels increases, the binarization processing becomes more complicated and the amount of calculations increases.
Further, the memory capacity required for the H-V conversion increases as the printer resolution and the width that can be printed in one pass increase or, in a more general term, as the number of nozzles in the print head increases. This gives rise to another problem that when the resolution is enhanced, the memory capacity required for processing increases in proportion to the square of the resolution.
Further, high-resolution printers tend to have an increased number of nozzles integrated in the print head and make ink droplets ejected from these nozzles smaller to reduce dot diameters and thereby realize higher resolutions. Since the ink droplets ejected from the nozzles of such a print head are very small, dot position variations due to landing deviations are apt to become large relative to the dot diameter. Thus even the multipass printing described above may not be able to eliminate image quality degradations caused by printing variations. Further, the multipass printing often uses a checker pattern as the mask pattern in the mask processing. When such a mask pattern is used, problems may arise in which dots that need to be printed fail to be printed or dot printing concentrates on a particular pass.